Optical parts, formed by using polymer materials, include optical lenses, such as micro-lenses, micro-lens -arrays, Fresnel-lenses, lenticular-lenses, prism-sheets, diffraction gratings, non-spherical lenses, camera-lenses, glass lenses, and optical communication parts, such as optical fibers, optical waveguides and optical switches. In any of these optical parts, it is important to control the refractive index, and among these, with respect to the optical waveguides, the refractive index should be precisely controlled to a level of the third digit or more below the decimal point. Since the precise control of the refractive index is influenced not only by the polymer materials, but also by forming methods of the optical waveguide, various polymer materials for use in optical waveguides as well as many forming methods for optical waveguides have been proposed. The forming methods are mainly classified as follows.
Selective polymerization method: A polymer-film (substrate) is impregnated with a monomer (the resulting polymer after a photo-polymerization has a refractive index smaller than that of the substrate) containing a photo-polymerization initiator, and only the clad portion of the resulting film is then irradiated with light through a photo-mask to undergo a polymerizing reaction. Thereafter, the monomer on the un-irradiated portion (core portion) is removed by using a solvent or the like so that an optical waveguide is formed.
Photolithography +RIE method: After an optical waveguide layer has been formed through photolithography, a core portion is formed therein through dry-etching, and an upper clad portion is applied thereto and formed thereon.
Direct exposing method: After a core portion has been formed through photolithography, an upper clad portion is applied thereto and formed thereon.
Bleach method: Only the clad portion, which corresponds to a desired portion of a polymer film, is irradiated with energy such as light through a photo-mask to undergo a chemical reaction so that the refractive index is changed; thus, a waveguide is formed.
Stamper method: Onto a monomer (lower clad portion) applied to a substrate, a recessed section is formed in a cured state by using a stamper, and a monomer is then injected into the recessed section to be cured to form a core portion, and an upper clad portion is formed thereon lastly.
Among these forming methods, it is considered that the stamper method is most prospective from the viewpoints of productivity and low costs, and a radically polymerizable acrylate-monomer or a photo-cationic polymerizable epoxy resin is suitable for this forming method.
For example, the use of a fluorinated epoxy resin or a fluorinated epoxy(meth)acrylate-monomer has been proposed (Japanese Patent Application Laid-Open No. 6-174956); however, since the hydroxide group is contained in the cured polymer, the resulting problem is that a great optical loss is caused in the wavelength band for use in light communication, in particular, in a band of 1550 nm. A straight-chain fluorine-containing (meth)acrylate-monomer, which contains no epoxy(meth)acrylate group, is also commercially available; however, this monomer is poor in heat resistance, and in particular, during an assembling process using solder under a high temperature, the polymer tends to be deformed and colored, and causes a problem of poor heat resistance, such as a thermal decomposition.
An active energy-ray curing-type composition for use in optical lenses, which is composed of a fluorine-containing compound, such as a fluorine-containing (meth)acrylate-monomer, and its homopolymer or its copolymer with another (meth)acrylate-monomer, and a non-fluorine polyfunctional (meth)acrylate-monomer, has been proposed (Japanese Patent Application Laid-Open No. 2001-74912). With respect to the non-fluorine polyfunctional (meth)acrylate-monomer, for example, aliphatic or aromatic polyvalent(meth)acrylates and dicyclopentenyl(meth)acrylate have been proposed. Although these proposals have achieved a low refractive index of a cured product (optical lens), a problem arises in which a heat-resistant property is lowered to cause a poor heat resistant decomposing property and a coloring-resistant property under a high temperature.
In order to solve the above-mentioned problems of the optical loss and insufficient heat resistance, a method, which uses fluorinated polyimide as a main skeleton to ensure a sufficient heat resistance, has been proposed (Japanese Patent Application Laid-Open No. 7-36068); however, an optical waveguide, made from a polymer which has a main skeleton composed of fluorinated polyimide, has the following problems:
(1) Insufficient moisture-resistant property,
(2) High birefringence due to an aromatic ring, and
(3) Difficulty in controlling refractive index.
As described above, in the conventional techniques, it has been very difficult to achieve both of a heat-resistant property of a polymer material, such as a heat resistant decomposing property and a coloring-resistant property under a high temperature, and a refractive-index control to a level of the third digit or more below the decimal point.